Very simply put, a good photograph is one that engages the viewer.
I've had a scan over the thread etc and knew things like story telling and technical ability would be mentioned. However, you can't tell a story in every type of photo, its impossible and a lot of the public have no idea about the technicalities of photography.
What engages the viewer might be different for each viewer. Photos like the Afghan Girl, the vietnam soldier shooting a man in the head (http://www.executedtoday.com/images/Nguyen_Van_Lem_big.jpg) or even Ansel Adams' landscapes will all make the viewer feel different things, and for others, they may feel nothing.
Personally I would take "classic" photos like those mentioned and go through them, ask the class what they feel from each photo and discuss them.
I agree with this. When you say "a good photograph is one that engages the viewer", I said a good photo is captivating, these are synonyms really.
I agree with he story telling. I mentioned technical abilities only in passing because most good photos have a mostly good technical side but by no means follow a list of rules and it is by no means a requirement. The great painters were mostly technically good, but its wasn't a requirement, and some styles are not classical in technique, such as Picasso's more abstract art.